This invention relates to a control device for a vehicle AC generator.
One example of a conventional control device for a vehicle AC generator is as shown in FIG. 2. In the case of the control device shown in FIG. 2, an AC generator 1 has an armature coil 101 and a field coil 102. A rectifier 2 for rectifying AC output voltages is connected to the AC generator 1. The rectifier 2 has a main output terminal 201, an auxiliary output terminal 202, and a ground terminal 203. One end of the field coil 102 is connected to a voltage regulator 3. The voltage regulator 3 comprises: voltage division resistors 301 and 302 for voltage detection; a control transistor 304 whose base electrode is connected through a Zener diode 303 to the connecting point of the two voltage division resistors 301 and 302; a power transistor 305 which is controlled by the transistor 304; a resistor 306; a suppression diode 307; and an initial exciting resistor 308.
The main output terminal 201 of the rectifier 2 is connected to a battery 4. The other end of the field coil 102 is connected to the auxiliary output terminal 202 of the rectifier 2, and it is further connected through the initial exciting resistor 308 and a key switch 5 to the positive terminal of the battery 4. A display lamp 6 is disposed in parallel with the initial exciting resistor 308.
The positive terminal of the battery 4 is connected through a load switch 8 to a vehicle electrical load 7.
The conventional control device thus designed operates as follows: When the key switch 5 is turned on, an initial exciting current flows in the field coil 102 through the parallel circuit of the display lamp 6 and the initial exciting resistor 308, so that the display lamp 6 is turned on. When the field current flows and the generator 1 is driven, a power generation is carried out, so that AC voltage is induced across the armature coil 101. As a result, the potential difference across the display lamp 6 becomes 0 V, so that the latter 6 is turned off.
The AC voltage induced in the armature coil 101 is rectified by the rectifier 2 and applied to the battery 4 to charge the latter 4. If, in this operation, the output voltage of the generator 1 is lower than a predetermined value, the voltage provided by the voltage division resistors 301 and 302 is also low. Accordingly, the Zener diode 303 is non-conductive, and the controlling transistor is also non-conductive, so that the power transistor is conductive. As a result, the field current flowing in the field coil 102 is increased, and the output voltage of the generator 1 is increased accordingly. When, on the other hand, the output voltage of the generator 1 is higher than the predetermined value, the voltage provided by the voltage division resistors 301 and 302 is also high. Hence, the Zener diode is rendered conductive, and the controlling transistor 304 is also rendered conductive, so that the power transistor 305 is rendered non-conductive. Therefore, the field current is decreased, and the output voltage of the generator 1 is decreased accordingly. The above-described operations are repeatedly carried out until the output voltage of the generator 1 reaches the predetermined value. The suppression diode 307 absorbs the surge induced on the field coil 102.
When, in the above-described conventional device, the power transistor 305 is short-circuited, or the end of the field coil 102 which is to be controlled in an on-off mode is grounded with metallic foreign matter or the like, the on-off control of the field current cannot be performed any longer, so that the field current is allowed to flow continuously. As a result, the output voltage of the generator 1 is raised abnormally high, and the battery is charged excessively. The display lamp 6 gives no alarm for this abnormal condition. If this over-charging occurs during high speed operation, and lasts for a long time, then a large quantity of hydrogen gas is produced from the battery electrolyte which in the worst case may explode the battery at worst. During driving of the vehicle at night, the excessively high voltage may burn out the filaments of the head lamps, thus making it impossible to drive the vehicle. In addition, the excessively high voltage may break the engine controlling computer, thus making it impossible to control the engine.